Absorptive gas microwave measuring system



S5-612 Au 233 EX l HPM-35 XR 2,423,383/

July l, 1947- w. D. HERSHBERGER 2,423,383

ABSORPTIVE GAS ICROWAVE MEASURING SYSTEM original Filed June 15, 1944INVENTOIL 0 T MAIVEY Patented July 1, 1947 ABSORPTIVE GAS MICROWAVE'MEASUBING SYSTEM,

William D. Hershberger, Princeton, N. J., assignor to Radio Corporationof America, a corporation of Delaware Original application June 15,1944, Serial No. 540,428. Divided and thisappllcation February 13, 1945,Serial No. 577,709

This application is a division of my copendins application Ber. No.540,428, filed June 15, 1944,

entitled "Light valve," and assigned to the same assignee as the instantapplication.

' This invention relates generally to microwave transmission and moreparticularly to improved methods of and means for controlling the deflection of a lightbeam by varying the refractive index of a microwaveabsorptive gas interposed'in the path of the light beam.

'I'he invention utilizes the characteristics of some gases which aresubstantially perfect'dielectrics at most radio frequencies butwhich-absorb considerable energy at certain other predeterminedmicrowave frequencies. For example, in an article by Cleeton andWilliams in Physical Review 45, 234 (1934) observations on microwaveabsorption in ammonia gas indicated that radiation having a wavelengthof 1.25 centimeters will lose approximately 63 percent of its initialenergy upon passing through 1.1 meters of ammonia. gas in a non-metalliccontainer at atmospheric pressure. It was noted further that theabsorption frequency band is relativelywide since the4 absorptioncoeiilcient falls to approximately onehalf ofy its maximum value atwavelengths of 1 centimeter and 1.5 centimeters. The observationsdescribedin the article identified heretofore were inspired by earliergeneral theoretical work on the energy levels of the ammonia moleculetogether with observations on the infra-red spectrum of this gas, but inall such prior experiments no attempt was made to determine, ex-

plain or utilize the eil'ect upon the gas of theV microwave absorptionby said gas.

Y 'I'he instant invention is related to the invention described inapplicants copending application Ser. No. 537,960, filed May 29, 1944,wherein the change in pressure of a microwave absorptive gas in responseto microwave irradiation is utilized to provide a novel method of andmeans for measuring microwave energy as a function of the change, orrate of change, of the pressure of said Sas.

The present invention utilizes the variation in the refractive index ofa microwave absorptive -gas in response to pressure changes therein dueto microwave irradiation, to provide a novel and emcient light valve. Ineffect, the invention com prises a novel light prism, the refractiveindex of which may be directly controlled as a function of microwaveirradiation.

It is believed that the variation in refractive index of a microwaveabsorptive gas subjected to predetermined microwave irradiation is dueto 8 Claims. (Cl. 171-95) heating of the gas by molecular resonanceeects incidental to the excitation of the energy levels of the gasmolecules. It is known that the microwave energy absorption in the gasincreases as a function of the gas pressure. The variation in refractiveindex of a light valve of the microwave absorptive gas type also isincreased as a function of the intensity or concentration of themicrowave irradiation of said gas.

A preferred embodiment of the invention comprises a cavity resonatorhaving a microwave permeable window opening into a waveguide terminedregion of the interior of the cavity resonator. Microwave absorptivegas, such. for example, as ammonia, is introduced into the cavity 20-resonator at the desired pressure to provide the the conductiveprojecting elements are triangular required sensitivity. Light permeablewindows disposed on opposite faces of the cavity resonator adjacent theconductive projecting elements permit' the focusing of a light beamthrough the cavity resonator between the conductive projecting elementswhereby the light beam penetrates the gas through a region subjected tothe maximum electric field within the cavity resonator. If

in cross-section or rectangular with faces other than parallel to thelight permeable windows, the microwave field between the conductiveelements provides a region having a relatively abrupt change inrefractive index which will deflect or refract a light beam directedtherethrough.

Any well knownmeans may be employed for introducing and venting thedesired microwave absorptive gas in the cavity resonator. Also, ifdesired, the cavity resonator may be tuned to the operating microwavefrequency by any conventional tuning screw, tuning plug 0r other knownmeans. Similarly. `by means of additional adjustable reactive elementssuch as tuning screws or tuning plugs disposed in the waveguide adjacentthe window into the cavity resonator, the microwave resonator impedancemay be matched to the characteristic impedance of the waveguide 69microwave ahsorptive gas chamber, thereby pro- -of the light beam 'maybe employed in cooperation atively sharply tuned cavity resonators. andis ,of high intensity since practically alll of the mif crowave energyis absorbed within the resonator. The cavity resonator may be tuned tothe desired applied frequency by means of a tuning plunger. or tuningscrews, of any type well known in the art. The tuning adjustments may bemade through gas-tight gaskets.in the with a fixed aperture device tovary the amount of iight or the position of the light beam which isfocused upon the oscillographic screen or moving photographic nlm.

Among the objects of the invention are to provide an improved method ofand means for measuring microwave energy. Another object of theinvention is to provide a novel method of and means for controlling thelightrefxractive index of a microwave absorptive gas. Anaddicavityresonator wall. or by means of Sylphon joints common to conventionalvacuum systems. Similarly. the various microwave or light permeablewindows in the cavity resonator walls may be made gas-tight byconventional rubber tional object of the invention is to provide animproved method of and means for controlling i the refraction ordeflection of a light beam in response to the magnitude of microwaveirradiation of a microwave absorptive gas.

Another object of the invention is to provide an improved method of andmeans for controlling a light beam. A further object of the invention isto provide an improved -light valve which may be employed in cooperationwith a ilxed aperture device to provide convenient and emcient means formodulating a light` beam in response to the modulation of a sourceofmicrcwaves. A further object of the tinventionis to provide animproved microwave wattmeter. still further object of the invention isto provide an improved method of and means for recording sound ona,moving motion picture film comprising a microwave responsive lightvalve,

means for focusing a light beam through said light valve and through afixed aperture device y to said nim. and means for varying therefractive index of said light valve in response to modulated microwavescharacteristic of the sound to be recorded upon said film.

'Ihe invention will be described in greater detail by reference to theaccompanying drawing of which Figure 1 is a schematic plancross-sectional view. taken along the section line I, I of oneembodiment of the invention. Figure 2 is a cross-sectional elevationalview taken along the section line II, Il of said embodiment of theinvention. Figure 3 is a schematic plan cross-sec- .tional view of asecond embodiment of the invention adapted te. the recording of sound onnlm,

A Figure 4 is a fragmentary view of a fixed aperture plate and a movablenlm forming a portion of the system of Figure 3, and Figure 5 is aschematic plan cross-sectional view of a third em bodiment of 'theinvention. Similar reference characters are applied to similar elementsthroughout the drawing.

Either tuned or untunedcavity resonators. into which predeterminedmicrowave energy absorbent gases may be introduced at predeterminedpressures. may be employed to conilne the active element to providevariations in the refractiveindexuof the light valve. For the purpose ofilgaskets, held under suitable pressure. Also.

metal-to-glass seals such as are employed in the fabrication' of vacuumtubes used in radio broadcast equipment may be utilized.

Reactive tuning elements coupled to the waveguide intermediate thesource of microwave energy and the cavity resonator may be `em, ployedto provide proper matching of the resonator to the transmission systemsurge impedance in order substantially to prevent wave re- Iiections andthereby to'insure that mostyof the transmitted energy is confined to thecavity resonator and absorbed by the gas therein.

An untuned cavity resonator of the type illustrated in Fig. 3 isproportioned so that, in view of the Q of the device-which is determinedby the resonator wall losses and the losses in the ammonia gas-theresonant modes are so closely spaced as to overlap. This condition maybe achieved by selecting the volume of the resonator to be larger thansome minimum value in view of the expected Q of the resonator.

The number of resonator modes An lying in the frequency range Af, whichis determined in turn by the value of Q in the relation @I l f .f Q isapproximately (l) g An-SA'T where Vo is the volume of the resonator andM is the wavelength of the microwave energy.

The variation in temperature AT of the microwave absorbent gas withinthe cavity resonator may be calculated from the. gass pressure Ap by therelation 2) Les where p is the static pressure and T is the absolutetemperature.

Hence. in accordance with known relations, it will be understood thatthe variation in the refractive index of the enclosed gas will be afunction of the variation in pressure of the gas in response to absorbedmicrowave energy. Since the refractive index of the gas also will becontrolled indirectly by low losses in the cavity resonator, and by theheat transferred from the gas to the cavity resonator walls, it may befound to be desirable to thermally insulate or to control, in any knownmanner. the temperature of the resonator walls.

'I'he energy directly absorbed by the gas from the microwave'transmission system provides a substantially rapid increase in gastemperature. and hence, in the refractive index of the gas, since thegas has a relatively low heat capacity and a relatively high temperaturecoeilicient of expansion. These features, therefore. will providerelatively high variations in the refractive 5 index of the enclosed gasin response to modulated microwave energy irradiating said gas. Unlessthe cavityresonator walls are thermally insulated from the enclosed gas,or are maintained at substantially constant temperature, by means of anair blast or other known expedient, the heat .transfer between thecavi-ty walls and the enclosed gas may provide relatively slow,protracted pressure variations in the gas which will be reflected j, asundesiredvariations in the refractive index system, performssubstantially as a perfectly matched load which absorbs all of themicrowave energy introduced thereto. Since, as explained heretofore,conductive elements connected to the inner walls of the cavity resonatormay be employed to concentrate the electric ileld within the resonatorin a. predetermined desired region, the overall emciency of the devicemay be adjusted to a relatively high value. As will be explained ingreater detail hereinafter, the resultant microwave absorptive gas-tightlight valve may be designed to provide single or multiple angular orlateral detlections of an applied light beam, which may be utilized inany known manner to provide desired indications of the variations in therefractive index of the gas.

Referring to Figure 1 of the drawings, a conventional rectangularwaveguide I opens, through an aperture 9. into a cavity resonator 5. lAgastight, microwave-permeable window 9 covers the aperture 9; Aninwardly projecting conductive element 1, having triangularcross-section, is fastened -to the lower inner wide face of the cavityresonator E. A similar inwardly-projecting, triangular cross-sectionalmember 9. shown in Fig. 2, cooperates with the lower projecting element1 to forma gap I I providing a region having a concentrated electricfield in response to microwave energy introduced into the cavityresonator from the waveguide.

Apertures I3, Il, in opposite side faces of the cavity resonator l, arecovered by gas-tight, lightpermeable windows I1, I 9, respectively. Alight source 2I, which may include a condensing lens system 29, directsa light beam, indicated by the dash lines 25 or 21, through the windowsin the cavity resonator 5, transversely of the gap I I between theinwardly projecting conductive elements 1 and 9.

Microwave energy from a microwave generator (not shown) is applied tothe cavity resonator lr through the waveguide I and themicrowavepermeable window 9 to establish a microwave neld within thecavity resonator which is substantially concentrated in the gap I Ibetween the conductive elements 1 and 9. The cavity resonator I enclosesa microwave absorptive gas, such, for exampleas ammonia. The gas may bemaintained at any desired pressure to provide the desired sensitivity.The concentrated microwave field established between the inwardlyprojecting elements 1 and 9 provides a region of increased gas pressurein the gap II due to heating of the gas from the absorbed microwaveenergy incident to the concentration ofthe electric eld in said gap. Theincreased gas pressure in the gap II 4 6 i, results in a correspondingvariation in the light refractive index of the ammonia gas in said gap.Hence, the boundary region between the gas in -the gap II and the gas inthe remainder of the cavity resonator 5 effectively comprises a sasprism of which the refractive index may be varied as a function of themagnitude of the microwavev energy introduced into the cavity resonator.

Since the refractive index of the prism may be varied as a function ofthe applied microwave energy, the light beam directed through the gasprism will be deflected, for example, as shown in the dash lines 25 or21. If desired, a projection lens 29, disposed adjacent the outputwindow I9, may be employed to focus the light beams 2i ,or

Y 21 upon a' suitable calibrated scale 9|, .whereby the microwave powerabsorbed in the cavity resonator i may be indicated directly.

Referring to Fig. 2, the cavity resonator I may `be tuned to resonate tothe applied microwave energy by means of a conventional tuning screw 23,which preferably includes a gas-tight rubber gasket 95. The ammonia gasmay be introduced into the cavity resonator 5 through aconventional gasvalve 31, disposed adjacent a gas intake aperture 99 in one of the wallsof the cavity resonator i. In order to match elfectively the limpedanceof the cavity resonator 5 to the surge impedance of the waveguide I toprevent wave reflections from th'e cavity resonator to the micro- Y waveenergy source, tuning screws 4 I, I3 or other v disposed on thewaveguide walls adjacent to the microwave permeable window I.

It should be understood that while the device disclosed in Figs. 1 and 2is described as a wattmeter for the direct measurement of microwaveenergy, that a similar device may be employed to record the modulationcharacteristics of microwave energy upon a suitable moving screen orphotographic film for cillographic or" related purposes. A

Referring to Figs. 3 and 4. a light valve is illustrated which providesa parallel displacement of a light beam in response to variations in themagnitude of microwave energy introduced into a gas-filled cavityresonator. In this second embodiment of the invention, the structure maybe :dennen to that described heretofore in Figs. 1 and 2, with theexception that the inwardly projecting conducting elements disposedwithin the cavity resonator are substantially rectangular incross-section, and are disposed in a manner whereby their side facesform acute angles with the light permeable windows I1, I9. A- vlightbeam, derived from the light source 2I and condenser lens system 29, isintroduced into the cavity resonator B through the input light windowI1, to provide parallel displacement of the light beams 25 or 21 inresponse to different magnitudes of microwave energy introduced into thecavity resonator.

It desired, the parallel displaced light beams 25 or 21, which passthrough the output light window I9, may be focused by means of theprojection lens 29 upon an apertured mask I5. Light which penetrates theaperture 41 in the apertured mask 45 may, if desired, be focused bymeans of a supplementary projection lens system 49 upsoln the desiredarea of a moving photographic nlm The device thus described may beemployed for the purpose of recording signal modulation upon a movingphotographic film such, for example, as in motion picture soundrecording.

s,4as,saa

lated in any known manner by means of the desired modulation signals toprovide a lightbeam of variable area, which may be recorded upon themoving nlm di. The dashed circles l, il, adjacent the aperture lill inthe apertured plate II, indicate the manner in' which the light beamderived from the cavity resonator light valve is masked by the maskingplate 4I. It should be understood that, alternatively, the device'4described in -Fig. 3 may be used for the direct measurement ofmicrowave power by focusing the output light beam upon a suitablycalibrated scale. as described heretofore in Figs. 1 and 2.

Referring to Figure 6, a third embodiment oi the invention isillustrated wherein the inwardlyprojecting conductive. elementsdescribed heretofore are omitted, and wherein the light permeablewindows I1, I 9 are disposed at current nodes in opposite walls of thecavity resonator 5. In this third embodiment of the invention the cavityresonator 5 comprises a section of the waveguide I, having an effectivelength of three half-wave'- lengths. The tuning screw 3l may be locatedon one of the side walls of the cavity resonator, 5 or, if desired, maybe disposed in the end wall' focsed'b'ythe projection lens 29 upon'calibrated scale 3|, for the direct measurement of the magnitude of themicrowave energy introduced into the cavity resonator.

Alternately, the latter device may be employed. as described heretoforein Figs. 3 and 4, for the recording of light variations upon a movingphotographic lm.

It should be understood that the sensitivity of the embodiment of theinvention illustrated in Fig. 5 will be less than that of theembodiments described in Figs. 1, 2, and 3f since the electric fieldl.within the cavity resonator .not be concentrated in the region throughwhich the light beam is directed. However, the embodiment shown in Fig.5 provides a convenient construction, especially in instances whereinconventional 1.25 centimeter waveguides are employed, since the physicalsize of such waveguides seriously limits the use of internal structuralelements. Furthermore, the embodiment of the invention illus trated inFig. 5 has the advantage that the light permeable windows I1, I9 aredisposed at points of low magnetic eld intensity, thereby minimizingmicrowave energy leakage from the cavity resonator i.

It should be understood that the impedance of the cavity resonator 5 maybe matched to the surge impedance of the waveguide I in the same manner'as described heretofore in Figs. l, 2 and 3, in order to minimize wavereflections from the cavity resonator to the microwave generator.

Various other microwave absorptive gases have been tested and found tobe quite satisfactory for microwave power measurements in apparatus ofthe type described heretofore. The following table discloses themicrowave frequencies at which some of these various gases have beenfound to absorb considerable microwave energy as indicated by theabsorption coeillcients which have been measured:

Power Abso tion Cou cient per Ethyl Chloride 'Ethylene Oxide.

embodiments of an improved light valve which may be employed for themeasurement or indication of microwave energy or signal modulation ofsaid energy. The devices disclosed provide an extremely convenient,accurate. and sensitive means, for measuring or indicating microwaveenergy inthe millimeter and centimeter regions, wherein the inventioneffectively comprises a gas type prism. the refractive index of whichmay be varied as a function of the magnitude applied microwaveenergy.

I claim as my invention:

l. A microwave energy responsive indicator including a microwaveresonant chamber for enclosing a microwave energy absorptive gas, meansfor introducing microwave energy to be measured into said chamber toestablish a microwave held in said gas for varying the light refractiveproperties of said gas, a light beam. source, an indicating lighttarget, and means for directing said light beam through said variablyrefractive gas to impinge upon said target in a manner whereby said beamis refracted as a function of the intensity of said field.

2. A microwave energy responsive indicator 1ncluding a microwaveresonant chamber for enclosing a microwave energy absorptive gas, amicrowave permeable window in said chamber, means for introducingmicrowave energy to be measured through said window into said chamber toestablish a microwave field in said gas for varye ing the lightrefractive properties of said gas in response to the energy absorbed bysaid gas, a light beam source, an indicating light target, and means fordirecting said light beam through said. variably refractive gas toimpinge upon said target in a`manner whereby said beam is refracted as afunction of the intensity of said ileld.

3. A microwave energy responsive indicator including a microwaveresonant chamber for enclosing a microwave energy absorptive gas, amicrowave permeable window in said chamber, means for introducingmicrowave energy to be measured through said window into said chamber toestablish a microwave field in said gas for varying the light refractiveproperties of said gas in response to the energy absorbed by said gas, a

light beam source. a calibrated light target, and

means for directing said light beam through said variably refractive gasto impinge upon said target in a manner whereby said beam is refractedas a function of the voltage induced across said chamber by said field.

4. A microwave energy responsive indicator including a microwaveresonant chamber for enclosing a microwave energy absorptive gas, a

microwave permeable window in said chamber,

a pair of conductive elements extending toward T r i 9 each other fromopposite sides of said chamber and forming a gap between adjacent endsthereof, means for introducing microwave energy to be measured throughsaid window int-o said chamber to establish a microwave eid in said gasin said gap for varying th'e light refractive properties of said gas insaid gap in response to the energy absorbed by said gas, a, light beamsource, an indicating light target, and means for directing said lightbeam through said variably refractive gas in said gap to impinge uponsaid target in a manner whereby said beam is retracted as a. function ofthe voltage induced across said gap by said field.

5. Apparatus as described in claim 4 including a lens disposedintermediate said chamber and said target for focusing said retractedlight beam on said target. i

6. Apparatus as described in claim 3 including reactive means for tuningsaid chamber to vary the light refractive characteristics of said gas inthe path of said light beam through said chamber.

WILLIAM D. I-IERSI-IBERGER'.

REFERENCES CITED FOREIGN PATENTS cuntry Date Germany Apr. 26. 1941Number

